With an aim to ensure data throughput of a terminal in a radio communication, it is essential to widen a radio band, and for the purpose of widening the band, research and development as well as standardization has been actively conducted on a multicarrier communication method.
In 3GPP (3rd Generation Partnership Project being a radio communication standards body, standardization of the specification called “LTE (Long Term Evolution)” has been advanced.
The LTE applies an OFDM (Orthogonal Frequency Division Multiplexing). The OFDM is a multicarrier communication method for generating a data signal in a frequency region, which is a method in which the data signal that has been converted into a data signal in a time domain by inverse Fourier transform is transmitted through a radio interval. A receive side in the LTE conducts compensation, demodulation and decoding of a channel variation after the data signal converted into the time domain has been returned to the data signal in the frequency domain by Fourier transform (for example, refer to 3GPP TSG RAN, “Physical Channels and Modulation (Release 8)”, 3GPP TS 36.211 ver 8.4.0, pp. 41-73, 2008/9).
Also, there is also applied a communication method of spatially multiplexing and transmitting the data signal at the same time and at the same frequency, called “LTE MIMO (Multi Input Multi Output)”. The MIMO is a method in which a transmitter station and a receiver station use a plurality of antennas, and data is spatially multiplexed and transmitted with the number of ranks of a propagation channel response formed between the transmitter and receiver antennas of the transmitter station and the receiver station as the upper limit. In the case of using the MIMO, the throughput of the terminal is improved as much as the data spatially multiplexed and transmitted, ideally times larger than the number of ranks (for example, refer to 3GPP TSG RAN, “Physical Channels and Modulation (Release 8)”, 3GPP TS 36.211 ver 8.4.0, pp. 41-73, 2008/9).
Also, as the same multicarrier communication method as that of the OFDM, there is an MC-CDMA (Multi Carrier Code Division Multiple Access) standardized by 3GPP2 being the standards body. This method is a method in which CDMA signals in a narrow band are bundled together in a plurality of carriers to apparently realize a wideband communication, which is standardized as spreading rate 3 (for example, refer to 3GPP2, “Physical Layer Standard for CDMA 2000 Spread Spectrum Systems Release A”, 3GPP2 C. S0002-A, Version 6.0, pp. 3-22-3-34, 2002/2).
In the radio communications system in which a plurality base stations communicate with terminals at the same frequency, there arises a problem of an intercell interference between the base stations. Also, there arises a problem that the communication quality of a terminal located on the boundary of cells produced by the base stations is degraded. To cope with those problems, a network-MIMO that conducts a communication by using the above-mentioned MIMO in cooperation with the plurality of base stations has been actively discussed (for example, refer to Laurence Mailaender, “Indoor Network MIMO Performance with Regularized Zero-Forcing Transmission”, IEEE ISSSTA 2008, pp 0.124-128, 2008/8). The network-MIMO eliminates the intercell interference being the problem with the conventional cellular system.
A manner of solving the problem by the network-MIMO executes signal processing so as to deal with a signal of another cell, which has been conventionally an interference signal with respect to a terminal that communicates with an arbitrary base station, because a component of the signal that has been conventionally dominant interference becomes a component of the desired signal, the communication quality of the terminal that is particularly located at the cell boundary is remarkably improved. In this example, there is a need to cooperate signal processing with data flow between the base stations.
With an aim to facilitate cooperation between the base stations for the above-mentioned network-MIMO, a cellular system has been proposed as a system for an IMT-advanced generation by the standards body 3GPP (for example, refer to NTT DoCoMo, “Inter-cell Radio Resource Management for Heterogeneous Network”, 3GPP TSG RAN WG1, R1-083019, 2008/8). FIG. 38 shows the overview picture of the cellular system disclosed in “Inter-cell Radio Resource Management for Heterogeneous Network”.
FIG. 38 is an explanatory diagram showing the cellular system in which baseband modems are aggregated at one place in the related art.
The cellular system shown in FIG. 38 includes cells 101-1 and 101-2, terminals 102-1 to 102-6, baseband modems 103-1 and 103-2, optical fibers 104-1 and 104-3, and front end portions 105-1 to 105-8. The cells 101-1 and 101-2 are represented by a cell 101, the terminal 102-1 to 102-6 are represented by a terminal 102, the baseband modem 103-1 and 103-2 are represented by a baseband modem 103, the optical fiber 104-1 and 104-3 are represented by an optical fiber 104, and the front end portion 105-1 to 105-8 are represented by a front end portion 105.
The feature of the cellular system shown in FIG. 38 resides in that the baseband modems 103 that have been conventionally allocated to the cells 101 at 1:1 are aggregated at one place. The aggregation of the baseband modems 103 at one place enables the above-mentioned cooperation that has been conventionally conceived to require communication at a distant place to be implemented by inter-substrate communication or inter-chip communication within the same rack. As a result, it is conceivable to ease cooperation between the base stations for the network-MIMO.